Effects of Al³⁺ on the microstructure and bioflocculation of anoxic sludge

The aluminum ions generated from mining aluminum, electrolytic aluminum and the industrial production of aluminum-based coagulants (such as AlCl₃ and Al₂(SO₄)₃) enter sewage treatment plants and interact with activated sludges. An anaerobic/anoxic/oxic (A²O) process was used to reveal the effects of Al³⁺ on the pollutant removal efficiencies, bioflocculation and the microstructure of sludge. The results showed that a low concentration of Al³⁺ improved the pollutant removal efficiencies and increased the sludge particle size. However, a high concentration of Al³⁺ hindered microbial flocculation and reduced the pollutant removal efficiencies. With a 10 mg/L Al³⁺ addition, the chemical oxygen demand (COD), total nitrogen (TN) and NH₄⁺-N increased by 3%, 16% and 27%, and reached as high as 68%, 60% and 87%, respectively. At the same time, the dehydrogenase activity, flocculation ability (FA) and contact angle of the sludge reached their maximum levels at 41.3 mg/L/hr, 45% and 79.63°, respectively. The specific surface area of the sludge decreased to 7.084 m²/g and the sludge pore size distribution shifted to concentrate in the mesoporous range. Most of Al³⁺ was adsorbed on the surface of sludge, changing the physicochemical properties and physical structure of the sludge.